With the commencement of the COP21 summit where global leaders have gathered in Paris to discuss about the solutions to global warming, two of the biggest cities became blanketed by endless smog.

The capitals of the world's two most populous countries, China and India, were covered by hazardous and choking smog Monday. While Beijing authorities took no time to issue an “orange alert” warning its people to stay indoors and cease all outdoor activities immediately, no such warning was issued in New Delhi.

Following the orange pollution alert in Beijing, which is the second highest levels, all the activities on the highways and construction sites were halted. Meanwhile, tens of thousands of people decided to overcome the pollution by participating in the Delhi Half Marathon.

On Monday, Beijing's particulate pollution level crossed 600 micrograms per cubic meter, more than 24 times the permissible threshold by the World Health Organization (WHO). The situation was no different in New Delhi, which showed a reading of around 530.

In 2014, New Delhi was deemed the world's worst polluted city by WHO. In addition, a study led by U.S. and Indian researchers revealed that the quality of air in the city is far worse than originally thought.

Meanwhile, the Ministry of Environmental Protection in China claimed that unfavorable weather conditions were behind the choking pollution blanketing the city. In winters, emissions get accumulated over northern China after the urban heating systems are switched on. In addition, the presence of low winds indicates that the polluted air stays constant and does not get dispersed.

The Chinese President Xi Jinping and the Indian Prime Minister Narendra Modi are in Paris to attend the climate change summit. The two-week long negotiations are expected to bring a consensus among nations regarding a policy to curb global warming.